A WATER-STABLE AND STRONGLY LUMINESCENT SELF-ASSEMBLED NONCOVALENT LANTHANIDE PODATE

The segmental ligand n-2-yl)-1,1'-dimethyl-2'-(5-methylpyridin-2-yl)-5 , benzimidazole) (L-9 reacted with an equimolar mixture of Ln(III) (Ln = La or Eu) and Zn-II in basic conditions to give selectively the sel f-assembled dinuclear non-covalent podates [LnZn(L-9-H),(3)](2+). Elec trospray mass spectrometry and proton NMR spectroscopy show that [LnZn (L-9-H),(3)](2+) adopt the expected head-to-head triple-helical struct ure with Zn-II pseudo-octahedrally co-ordinated by the bidentate bindi ng units of the three segmental ligands and Ln(III) occupying the rema ining facial pseudo-tricapped trigonal prismatic site produced by the wrapped unsymmetrical tridentate units. Upon UV irradiation, solutions of [EuZn(L-9-H),(3)](2+) in acetonitrile or in water produce strong r ed luminescence. The Eu (D-5(0),) lifetime and quantum yield indicate that Eu-III is efficiently protected from external interactions for co mplex concentration in the range 10(-4)-10(-8) M and that no solvent m olecule enters the first co-ordination sphere. Electrospray mass spect rometry combined with high-resolution emission spectroscopy confirm th at the structure of the dinuclear triple-helical complex [EuZn(L-9-H), (3)](2+) is maintained at low concentration which strongly contrasts w ith the lipophilic analogous non-covalent lanthanide podates [EuZn(L-i ),(3)](5+) {i = 7 or 8; pyridin-2-yl)-5,5'-methylenebis(1H-benzimidazo le)} which are decomplexed in acetonitrile for concentrations below 10 (-5) M. Detailed photophysical studies have established that [EuZn(L-9 -H),(3)](2+) works as an efficient UV-->VIS light-converting device in the solid state and in water.